linux-kernel - [RFC PATCH 09/13] vfio: pci: Fill only spec-defined configuration space regions

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Message-ID: <c1c437a9a2a146b302e1904d7af113e6ef4b0ac6.1759312886.git.epetron@amazon.de>
Date: Fri, 3 Oct 2025 09:00:45 +0000
From: Evangelos Petrongonas <epetron@...zon.de>
To: Bjorn Helgaas <bhelgaas@...gle.com>, Alex Williamson
	<alex.williamson@...hat.com>, "Rafael J . Wysocki" <rafael@...nel.org>, "Len
 Brown" <lenb@...nel.org>
CC: Evangelos Petrongonas <epetron@...zon.de>, Pasha Tatashin
	<pasha.tatashin@...een.com>, David Matlack <dmatlack@...gle.com>, "Vipin
 Sharma" <vipinsh@...gle.com>, Chris Li <chrisl@...nel.org>, Jason Miu
	<jasonmiu@...gle.com>, Pratyush Yadav <pratyush@...nel.org>, "Stanislav
 Spassov" <stanspas@...zon.de>, <linux-pci@...r.kernel.org>,
	<linux-acpi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<nh-open-source@...zon.com>
Subject: [RFC PATCH 09/13] vfio: pci: Fill only spec-defined configuration space regions

Ammend VFIO PCI configuration space initialization by filling only
the regions defined by the PCI specification, avoiding unnecessary
reads from undefined or reserved areas.

The current implementation reads the entire configuration space during
initialization, including reserved regions that may not be implemented
by the device or may have side effects when accessed. This change
modifies vfio_fill_vconfig_bytes() skips reserved regions in the
standard configuration space header.

This benefits the PCSC hit rate, as 4byte access to a region where only
2 bytes are cacheable and 2 are undefined, therefore uncached, will lead
to a HW access instead.

Signed-off-by: Evangelos Petrongonas <epetron@...zon.de>
---
 drivers/vfio/pci/vfio_pci_config.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/vfio/pci/vfio_pci_config.c b/drivers/vfio/pci/vfio_pci_config.c
index 8f02f236b5b4..4fc7156a77d1 100644
--- a/drivers/vfio/pci/vfio_pci_config.c
+++ b/drivers/vfio/pci/vfio_pci_config.c
@@ -1485,7 +1485,18 @@ static int vfio_fill_vconfig_bytes(struct vfio_pci_core_device *vdev,
 	while (size) {
 		int filled;

-		if (size >= 4 && !(offset % 4)) {
+		if (offset == PCI_CAPABILITY_LIST) {
+			u8 *byte = &vdev->vconfig[offset];
+
+			ret = pci_read_config_byte(pdev, offset, byte);
+			if (ret)
+				return ret;
+			/* Skip the reserved area */
+			filled = 4;
+		} else if (offset == 0x38) {
+			/* Skip the reserved area */
+			filled = 4;
+		} else if (size >= 4 && !(offset % 4)) {
 			__le32 *dwordp = (__le32 *)&vdev->vconfig[offset];
 			u32 dword;

-- 
2.47.3

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